Sen. Lowell P. Weicker Jr. (R-Conn.), who has been waging a lonely filibuster against anti-busing legislation, picked up important moral support yesterday as 11 senators joined him in signing a "Dear Colleague" letter saying the measure would "radically alter our basic constitional framework."

The letter, signed by six moderate Republicans and six Democrats, was seen as a warning signal to Senate conservatives that they can expect a floor fight on this and other pending bills that would clip the power of the federal courts.

It was sent to senators along with a strongly worded letter to Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) from the American Bar Association, which opposes, anti-busing amendments to a Justice Department authorization bill.

"This isn't a Weicker filibuster anymore," said one Weicker aide."It's a constitution issue."

For more than a week before the Senate adjourned for the July 4 recess, Weicker delayed a vote on anti-busing amendments by conducting what amounted to a one-man filibuster. The amendments, on which the Senate will resume work when it returns next week, are among the most stringent anti-busing proposals ever offered in Congress.

The first, offered by Sen. Jesse Helms (R-N.C.), would bar the Justice Department from taking part in suits that might "directly or indirectly" result in court orders to bus students for racial intergration.

This was vigorously opposed by civil rights groups. But a second amendment, authored by Sen. J. Bennett Johnston (D-La.), caused greater consternation. It would restrict all federal courts from busing students more than five miles or 15 minutes driving time beyond their homes.

The provisions would be retroactive. This, the 12 senators said in their letter, "would force the dissolution of remedies previously ordered by courts and long since implemented. It would reopen wounds that have been healed and could engender community conflict in many areas of the nation.

"The amendment intrudes on and erodes the independence of the federal judiciary by attempting to limit the remedies which may be required under the Constitution," the senators said. "Therefore it would radically alter our basic constitutional framework with regard to the manner in which the branches of our government relate to each other."

Republicans signing the letter were: Weicker, Charles H. Percy of Illinois, Mark O. Hatfield of Oregon, Charles McC. Mathias of Maryland, John H. Chafee of Rhode Island and Arlen Specter of Pennsylvania.

They were joined by Democratic Sens. Edward M. Kennedy (Mass.), Daniel Patrick Moynihan (N.Y.), Gary Hart (Colo.), Bill Bradley (N.J.), George J. Mitchell (Maine), and Spark M. Matsunaga (Hawaii).

The names on the letter were more important than its language. "In a nutshell, it means that there will be a floor fight on what Jesse Helms thought would be a relatively easy parliamentary move," said John Shattucks, legislative director of the American Civil Liberties Union. "When you have people signing the letter like Moynihan and Percy, who represent the moderate mainstream of their parties, it means you have significant political opposition.